Error in hair texture
It's likely the issue is causedby seeing the backside of your meshes Normals. This causes it to appear invisible if you aren't looking at the faces from the front. One way to prevent this is to take your mesh, duplicate it, then flip the normals so that you can see it from both sides. Here is some documentation explaining different ways of going about this. 

http://www.katsbits.com/tutorials/blender/double-sided-faces-different-materials.php

It is possible however, enable Double sided in Properties > Object Data > Normals. But this uses considerably more resources to render than a mesh alone.